Way TMI,,,,
If the plane is tailheavy, move the battery back more forward until it balances. On a Trainer like yours its going to balance on the wingspar.
1. Get someone to help you and find the wingspar, with the fuel tank empty each of you get on a wingtip and put your finger under the wingspar out towards the tip. Lift the airplane with you fingertip under the spar.
2. If the tail drops a little its tail heavy, if the nose drops its noseheavy.
3. Move the battery pack forward or back until the plane balances.
4. Thats it!

There are lots of possibilites that can cause your problems. CG and being out of trim are the two most likely. Theres a real good chance that you just need to add some down trim in the elevator.

Turn on the radio and reciever in the plane and look at the elevator from the end. The elevator and the horizontal stab should be in line with one another. If the elevator is deflected slightly upwards then either mechanically adjust it by shortening the pushrod till its straight, or if its just a little bit. GO ahead and add some downtrim to make it line up with the stab.

If you have a larger engine that recommended in your plane and you assembled it otherwise according to the instructions it is very unlikely, almost impossible that you are tailheavy.